Touch-sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touching object such as a finger or stylus, either in direct contact, or through proximity (i.e. without contact), with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on, e.g., hand held devices, such as mobile telephones, but also on larger devices and displays. A touch panel that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
There are numerous known techniques for providing touch sensitivity, e.g. by incorporating resistive wire grids, capacitive sensors, strain gauges, etc. into a touch panel. There are also various types of optical touch systems, which e.g. detect shadows cast by touching objects onto a touch surface, or detect light scattered off the point(s) of touching objects on a touch panel.
One specific type of optical touch system uses projection measurements of light that propagates on a plurality of propagation paths inside a light transmissive panel that defines a touch surface. The projection measurements thus quantify a property, e.g. power, of the light on the individual propagation paths, when the light has passed the panel. The light propagates inside the panel by total internal reflection (TIR) against the touch surface, such that objects on the touch surface causes the propagating light on one or more propagation paths to be attenuated, commonly denoted FTIR (Frustrated Total Internal Reflection). For touch determination, the projection measurements may be processed by simple triangulation, or by more advanced image reconstruction techniques that generate a two-dimensional distribution of disturbances on the touch surface, i.e. an “image” of everything on the touch surface that affects the measured property. Examples of such touch systems are found in U.S. Pat. No. 3,673,327, 4,254,333, 6,972,753, 7,432,893, US2006/0114237, US2007/0075648, WO2009/048365, US2009/0153519, WO2010/006882, WO2010/064983, WO2010/134865 and WO2012/105893.
WO2013/036192 discloses a light coupling structure for optical touch panels, such as of the type in the above referenced documents. The coupling structure is used to in-couple light from a light source, such as an LED, to the panel at an angle suitable for TIR (total internal reflection) in a touch panel. The light coupling structure is relatively large and takes up significant space underneath the panel. However, the available space for the touch-sensing system is scarce, particularly at the periphery of the touch panel where opto-electronic components are mounted in an electrical device. This is even more problematic for smaller devices having a touch-sensing system. The light coupling structure is also relatively costly to manufacture and mount on the panel. Finally, a reliability problem may occur as the temperature coefficient differences between components and the panel can result in reduced performance over time. Another problem is that, for optical components with a broad illumination directed onto to the glass, a large fraction of the light will not be coupled into the panel.
Attempts have been made to use a film with dome shaped lenses arranged in an array on a transparent substrate for coupling light from the light source to a panel of a touch-sensing system. Such substrates with dome shaped lenses are e.g. disclosed in WO2006/034409A2 but used for a different purpose than coupling light into a light transmissive panel of a touch-sensing system. In the field, the ‘region of interest’ is defined as the angular range, both in the theta (θ—i.e. the angle of the light from the normal of the plane of the panel) range and phi (φ—i.e. the angle of the light from the normal of the edge of the panel and in the plane of the panel) range of light travelling in the glass from which the system is configured to derive a touch signal. This range may be chosen for optimal touch resolution and to exclude contamination noise. In a touch-sensing system using TIR for the propagation of the light in the touch panel, the region of interest of light inside the panel is between 40°-90° for θ, although preferably between 50°-75°, and a range of ±75° for φ. This means that for a dome shaped structure, only a small fraction of the dome shaped surface refracts the light at an angle to propagate within the panel via TIR within the region of interest and to provide effective and contamination resistant touch detection. Therefore, a dome shaped solution is not efficient for in-coupling of light to a touch panel. Hence, a shape of the primitive that directs light needs to be found to couple larger numbers of photons into the light transmissive panel at angles matching the ROI for touch-sensing systems based on light propagating by TIR.
The present invention addresses a widely recognized need for efficient coupling of light into a light transmissive panel for a touch-sensing system, and thus provides for improved power efficiency and/or a more compact design.